High-speed optical data networks generally use semiconductor light sources including vertical cavity devices to generate the light used to carry optical signals through an optical medium from a transmitter to a receiver. The semiconductor light source is typically driven by an electronic chip that conditions electrical data into electrical drive waveforms for the light sources. The light sources then transmit the data in optical data waveforms. The receiver typically includes photodetectors to convert the optical data waveforms from the transmitter back to electrical signals that can then be amplified and converted into electrical data waveforms. High-speed optical data networks can transmit data at much high data speeds than possible with purely electrical data networks.
Optical data networks are used in military applications such as simulating weaponry, training soldiers, combat identification of friend or foe, and other applications. An example of such a training system used by the United States Army is the Multiple Integrated Laser Engagements System (MILES). Another such training system is the Optische Schnittstelle fur Agdus and GefUbz H (OSAG), translated Optical Interface for Force-on-force Training Simulator and Army CTC. These systems often use semiconductor lasers that send optical code through free space to a receiver and are often referred to tactical engagement systems.
Tactical engagement systems may also be used in gaming or other non-military systems. For use in soldier training by the military, the tactical engagement system includes a laser transmitter that is mounted on a weapon such as a small arms weapon or other military platform, a receiver that includes one or more detectors, electrical circuitry to control the laser and process electrical signals from the detectors, and other components such as barrel flash detector. For small arms weapons the laser transmitter is often referred to as a small arms transmitter or SAT. The laser transmitter may also be mounted on a larger or smaller military platform, such as tanks, trucks, helicopters or planes, and on smaller implements such as pistols.
For these tactical engagement applications the laser beam quality and the laser beam propagation properties through the atmosphere can be important in, as well as the receiver design to receive the signal through the atmosphere. Laser diodes now used in many military systems produce elliptical beams and asymmetric beam profiles, and have laser wavelengths that shift significantly between devices due to manufacturing tolerances, and due to internal temperature changes.
A particular problem in many training and battlefield applications that use tactical engagement systems is that the laser transmitters and receivers must be battery operated, and therefore operate efficiently. While the laser transmitter need only be activated during an engagement, the receiver must be ready to receive optical data at any time.
Optical data networks may also be formed using Light Detection and Ranging (lidar) for mapping or position sensing, which may transmit optical signals through free space to collect information regarding positions of distant or nearby objects. In this case a laser beam is reflected off a distant object, or multiple distant objects, and detected at a detector or detector array placed closer to the laser. The resulting optical data collected by the detector or detector array from the reflected laser pulses are used to map a distance, or multiple distances to reconstruct a scene, based on a technique such as time-of-flight or similar approach. Many lasers used for these systems produce speckle, which is an interference effect. Speckle degrades the signal returned from a target because it cause non-uniform reflections due to the coherence of the laser beam. A related problem occurs due to scintillation for laser beams that propagate long distances through the atmosphere. Scintillation can be viewed as an interference effect due to fluctuations in air pressure, wind, thermal gradients, or other effects that may influence the refractive index of the atmosphere.
A Vertical Cavity Surface Emitting Laser (VCSEL) is a laser resonator that includes two mirrors that are typically distributed Bragg reflector (DBR) mirrors which have layers with interfaces oriented substantially parallel to the die or wafer surface with an active region. The active region may include one or more bulk layers, quantum wells, quantum wires, or quantum dots for the laser light generation in between. The planar DBR-mirrors comprise layers with alternating high and low refractive indices. Each layer has a thickness of approximately one quarter of the laser wavelength in the material, or an odd integer multiple of the quarter wavelength, or in some cases even integer multiple of the quarter thickness, depending on layer placement and optical interference effects. The mirror layers can produce intensity reflectivities that may be above 99%, and in other cases may be produce much lower values of reflectivity. Slightly lower values than 99% can be useful to obtain high extraction efficiency of the laser light from VCSELs, and much lower values can be useful for RCLEDs or LEDs. Mirrors can also be made of other materials, including dielectrics or metals.
RCLED's are described in U.S. Pat. No. 5,226,053. A RCLED is a light emitting diode (LED) that generates mainly spontaneous emission and generally operates without a distinct threshold. The drive voltage of a spontaneous emitter can be less than its photon energy divided by the electron charge, under which condition it ideally absorbs heat in its light emission process. The RCLED's drive voltage can also exceed its photon energy, under which it generally generates heat in its light emission.
For high speed operation in optical data networks electrical parasitics and self-heating of the laser generally must be low. Therefore the design of the laser should have sufficiently low internal electrical parasitics combined with sufficiently low self-heating to enable high speed. For example, speeds of 50 GB/s or greater are desired in some applications, especially those that use fiber optics, and even greater data speeds in excess of 100 GB/s are of interest. These speeds are difficult to achieve if the electrical parasitics of the laser is too large, or self-heating is too great.
Another problem in optical data networks, high power pumping, and lidar is that at high power the laser diodes tend to operate in multitransverse modes. For a tactical engagement system, for example, this multimode operation can degrade beam quality and limit the system performance. In lidar the multimode operation can also degrade the signal, and require additional optics to.